I would call either 407-WDisney or 407-WDW-Dine and ask them to connect you to Grand Gatherings. It sounds like 10 of you may be staying on site with the
Disney Dining Plan. Is that correct? If so, the group of 10 can use Grand Gatherings to help plan events, so they ought to be able to help you plan the dinner for 40 at the Luau. They should also be able to put you in touch with the manager for help making arrangements.
Grand Gatherings can also help with other group events -- such as a special dessert event during Illumnations at Epcot. I find that the CM's working the Grand Gatherings phones are some of the most experienced so it pays to get through to them. I don't know whether they offer a group discount. Can't hurt to ask. I believe (others may be able to help on this) that using
DDP credits for the Luau means you can't get front row seating, so you may want to check on that, although with 40 people you should have enough "market power" to negotiate what you want!
I've always heard that brides can wear their wedding gowns at any restaurant that is not actually in a theme park. The Luau is held at the Polynesian Hotel, so that shouldn't be a problem, although you'll want to doublecheck.
The Luau is a fun show and meal, although there will be lots of other people and kids there so you won't have alot of privacy. The tables seat 10, so you'll have 4 tables to yourselves. When we went, we had a party of 10 so sat at our own table. We were fortunate to have a front row table. The dinner was tasty and the dancing in the show was quite impressive. The luau compares favorably with luaus we've done in Hawaii.
Other possibilities for dinner for 40 -- the California Grill has a private room that would accommodate 40 nicely, and you can view the evening's MK fireworks right from the room. The view of MK is gorgeous as well. The food is excellent with a nice wine selection as well. (Just me, but this would probably be my first choice if I were planning a post-wedding dinner for 40 simply because the setting is stunning and the food wonderful, plus you have the benefit of privacy for toasts, etc.)
Victoria & Albert's is very expensive, but lovely. They have a side room that can be made private, although I don't know if it accommodates 40.
Jiko has a private room that can accommodate 40 for dinner. No view from this private room. Jiko is at Animal Kingdom Lodge and is a fun and exotic place to visit. Guests can wander among the savannahs and view the animals pre-dinner.
I don't know if Citrico's has a private dining room, but it does have a corner area of the restaurant which, again, looks out on the MK and fireworks.
Just a few thoughts. Hope they're helpful. Good luck with your planning!